The invention relates to a diagnostic radiology system for angiographic X-ray examinations, comprising an image intensifier television pickup installation, at least one image memory, a subtraction device for the subtraction of image data occurring at different times, and a video monitor. Subtraction images find utilization in the case of fluoroscopic examinations, in particular in the case of selective angiography, in order to render blood vessels clearly visible which, in the normal X-ray image, can be poorly recognized or on which bone structures are superimposed.
In the German patent application No. P 31 22 098.3 (U.S. Pat. No. 4,483,342 issued Nov. 20, 1984), a diagnostic radiology system of the type initially cited is described in which a radiograph of a blank image (mask), that is an image taken without contrast medium filling, and of a filling image, that is an image taken with injected contrast medium, proceeds manually pursuant to visual control on the monitor. For storing in the image memory, a trigger must be actuated. In the German patent application No. P31 48 789.0 an image subtraction device is described in which the radiographic time-point for storage of the blank image, as well as the chronological interval of the stored blank image relative to the filling image to be stored are manually input into a program memory on an operating console. After a program start the radiographic image recording sequence proceeds automatically controlled by a control device. For the time-points of the respective radiographs, experimentally ascertained experimental values are utilized. Since, however, the time-point for the storage of an optimum mask and the time-point for the storage of an optimum filling image are substantially dependent upon the patient, in particular upon the blood flow velocity of the patient, in most instances one does not obtain any optimum subtraction images. In the case of the manual triggering of the storage operations pursuant to visual control, conditioned by the reaction time of the observer and by the low recognizability of the optimum time-points, the latter can be attained only in rare instances. In order to preclude a movement of the patient the radiographic exposure time-points must lie as closely as possible next to one another. However, this contradicts the demand for obtaining as great a contrast as possible in order to render possible the visibility also of small objects. A compromise between these two demands can be optimally found only with difficulty using the described storage methods.